1. Field of the Invention
The present invention relates to the production of heavy hydrocarbon products from light gaseous hydrocarbons such as natural gas, associated gas, coal seam gas, landfill gas, or biogas. In particular, the present invention relates to optimization of a catalyst for performing a Fischer-Tropsch reaction.
2. Related Art
Various processes are known for the conversion of carbonaceous feeds or light hydrocarbons containing gases into normally liquid products such as methanol, higher alcohols and hydrocarbon fuels and chemicals particularly paraffinic hydrocarbons. Such processes are directed at the objective of adding value to the feedstock by making a transportable, more valuable product such as diesel fuel or jet fuel or chemicals such as base oils or drilling fluids.
The Fischer-Tropsch process can be used to convert such feedstocks into more valuable easily transportable liquid hydrocarbon products and chemicals. The feedstock is first converted to synthesis gas comprising carbon monoxide and hydrogen. The synthesis gas is then converted to heavy hydrocarbon products over a Fischer-Tropsch catalyst. The heavy hydrocarbon products can be subjected to further workup by hydroprocessing such as hydrocracking and/or hydroisomerization and distillation resulting in, for example, a high yield of high quality middle distillate products such as jet fuel or diesel fuel. The heavy hydrocarbon products can also be upgraded to specialty products such as solvents, drilling fluids, waxes, or lube base oils due to the high purity of the Fischer-Tropsch products.
Processes that convert light hydrocarbons to heavier hydrocarbon products, for example, generally have three steps: 1) conversion of light hydrocarbon feedstock to synthesis gas comprising carbon monoxide and hydrogen; 2) conversion of the synthesis gas to heavy hydrocarbons via the Fischer-Tropsch reaction; and 3) hydroprocessing the heavy hydrocarbon product to one or more finished hydrocarbon products.
The design and optimization of the Fischer-Tropsch reactor is of paramount importance for the technical and economical success of a plant for the conversion of synthesis gas into hydrocarbons. A Fixed Bed Fischer Tropsch (FBFT) reactor is a very simple effective reactor that is very scalable.
A FBFT reactor must meet many conditions such as minimum complexity, ease of construction, minimum number of tubes, high selectivity towards desired products, high per pass conversion to avoid a second or a third stage, low pressure drop, etc.
It follows that the design of a FBFT reactor cannot be done without taking into account the characteristics and performance of the Fischer-Tropsch catalyst to be used.
This is a technical challenge that involves many variables. FIG. 1 is an attempt to visualize some of the main variables involved and to clarify their interaction.
For example, while it is known that a high activity catalyst is desired, this has an effect on the operating temperature (“T”). If the operating temperature is too high, the Fischer-Tropsch reaction rate will be high as well, increasing the possibility of a temperature excursion or run away. To avoid this, the reactor tube diameter (“D”) has to be smaller to facilitate the radial heat transfer. The overall heat transfer also increases with the gas linear velocity (“LV”), but increasing the linear velocity increases the pressure drop (“ΔP”) from the top to the bottom of the reactor.
To lower the pressure drop at any given set of conditions, the catalyst particle size has to increase, which may result in poor selectivities due to diffusion considerations. An alternative would be to lower the tube height, although this could decrease the per pass conversion (“PP CO conv”) of the carbon monoxide and, therefore, either increase the internal recycle or add another Fischer-Tropsch stage to reach the desired total CO conversion. This could also result in a larger number of shorter reactors therefore increasing the plant complexity and the capital cost. While taking all these considerations into account, the selectivity towards the desired products must not decrease.
It is therefore apparent that while the reactor design has to be easy to fabricate, it has to take into account the catalyst performance. The catalyst has to be large enough to minimize the pressure drop and to allow for an optimal reactor height based on a targeted CO per pass conversion, but not so large that it will cause a negative effect of the diffusion on the desired selectivity.
At the same time, the catalyst particle has to minimize the diffusion effect for any given particle size. This can be achieved by having pores of a diameter large enough so that the product's selectivity is not affected. On the other hand, a large pore diameter, associated with a pore volume large enough to accommodate a selected amount of Fischer-Tropsch active metal such as cobalt without narrowing the pore diameter too much and, therefore, cause diffusion problems, will weaken the mechanical strength of the catalyst particle. A low catalyst particle mechanical strength will cause catalyst breakage during the catalyst loading step and therefore increase the pressure drop to levels above design.
It is therefore necessary to develop a support for a Fischer-Tropsch catalyst that can meet all the above expectations in order to design a technically and economically viable Fischer-Tropsch process.
The current state of the art for fixed bed Fischer-Tropsch catalysts does not address all these issues and the technology described in the open art and in the patent literature is typically focused on a few of these considerations at a time, ignoring the negative effect that optimizing only one variable may have on the other variables.
Another problem in the art is the “open” optimization of the targeted parameters. That is, the optimized parameter ranges from a certain value to an infinite or zero value. In those cases, following the teachings may lead to technical failures.
Because of the above considerations, it is necessary to develop a Fischer-Tropsch Fixed Bed catalyst that can meet all the requirements for the construction of a technically and economically viable Fischer-Tropsch reactor for the selective conversion of synthesis gas into valuable hydrocarbon products. It is also desirable to develop a Fischer-Tropsch slurry bubble column reactor catalyst that can meet all the requirements for the construction of a technically and economically viable Fischer-Tropsch reactor for the selective conversion of synthesis gas into valuable hydrocarbon products.